There are three major subdivisions of our proposed studies in fine structure. First, pinocytosis will be investigated in the eye and olfactory epithelium of the snail Helix aspersa, using the tracer horseradish peroxidase combined with electron mircroscopy. We hope to determine the nature, extent and fate of the pinocytic vesicles. They knowledge obtained from these studies will enable us to understand better the function of two important sensory organs in a mollusk and to contribute to general biological information. Pinocytosis is a fundamental activity of cells that enable them to incorporate extracellular fluid in which various substances are dissolved or suspended. Second, neurological relationships of the eye, olfactory epithelium and brain of Helix will be studied using back-filling of nerve fibers with HRP or cobalt and serial sectioning combined with electron microscopy. These studies will also contribute to our understanding the functions of the dominant sensory organs of this mollusk. Third, evolutionary studies. There are three theories on the evolution of photoreceptors; the first one to be advanced is Eakin's hypothesis of two lines of phylogeny of organisms with ligh-sensitive cells: those groups of animals that have ciliary receptors and those taxa with microvillar (rhabdomeric) light sensors. Further testing of these theories is needed. We propose to examine the ultrastructure of photoreceptors in several invertebrates, especially their larvae, not yet studied with the electron microscope to help evaluate the validity of our theory. The first two projects are health related because of the potential contributions to fundamental biology upon which all applied biology rests.